....I'm a little worried about the size of shot to send through her M choke gun if it was steel....
What's the size limit for steel shot through a modified choke? Why not just run a little steel through it.
I do not know if there is a general rule, but when I had Briley put tubes in one of my doubles, they made it clear, =#4s was the max size for their chokes and the max choke for ANY steel was Modified.
Of course those are thin walled tubes and the steel in them is different than in the other chokes in the same set.
In this case, however, the chokes are fixed. I've always applied Briley's rule to all other guns, but I do not know if it is valid.